1. Field of the invention
This invention relates to a photovoltaic device module (solar cell module), and more particularly to a photovoltaic device module having high reliability against flexural fatigue at its electrically connected portions.
2. Related Background Art
In recent years, consciousness of environmental problems has spread on a worldwide basis. In particular, due to anxieties related to the phenomenon whereby CO.sub.2 emissions make the earth's environment warm, there is an increasingly earnest demand for clean energy. Solar cells are likely clean energy sources because of their safety and readiness in handling.
Solar cells have various forms. They are typified by:
(1) crystalline silicon solar cells; PA1 (2) polycrystalline silicon solar cells; PA1 (3) amorphous silicon solar cells; PA1 (4) copper indium selenide solar cells; and PA1 (5) compound semiconductor solar cells.
Of these, thin-film crystalline silicon solar cells, compound semiconductor solar cells and amorphous silicon solar cells are recently and extensively being developed in various fields because they can be made in a large-area at a relatively low cost. In particular, among these solar cells, thin-film solar cells typified by amorphous silicon solar cells produced by depositing silicon on a conductive substrate and forming a transparent conductive layer thereon, are considered promising as a form of modules for the future because they are light-weight and also highly impact resistant and flexible.
Usually, in battery-adaptable solar cells, a single-sheet solar cell alone does not have a sufficient output voltage. Hence, it is often necessary to use a plurality of solar cell devices connected in series. Also, in order to gain electric current quantity, solar cell devices are connected in parallel, and in some cases both the series connection and the parallel connection are used in combination.
An example of a photovoltaic device module will be described with reference to FIGS. 1, 2A, 2B, 3, 4A and 4B.
First, an amorphous type photovoltaic device will be described.
FIG. 1 is a diagrammatic plan view showing an example of the amorphous type photovoltaic device, as viewed on its surface (light-receiving surface) side.
In FIG. 1, reference numeral 201 denotes the photovoltaic device, comprising a substrate which supports the whole photovoltaic device and an amorphous semiconductor layer and an electrode layer which are formed on the substrate. The substrate is made of a metallic material such as stainless steel, and the semiconductor layer comprises a back reflection layer, an n-type semiconductor layer, an i-type semiconductor layer, a p-type semiconductor layer, an n-type semiconductor layer, an i-type semiconductor layer and a p-type semiconductor layer which are superposed in this order from the bottom layer by a film-forming process such as CVD, and is so set up that electric power is generated in a good efficiency when exposed to light. As the uppermost electrode layer, a transparent conductive film of indium oxide or the like is formed so as to serve also as an anti-reflection means and as an electricity collection means.
To form the transparent conductive film, an etching paste containing FeCl.sub.3, AlCl.sub.3 or the like is coated by a process such as screen printing and then heated so that the film is removed partly in lines along etching lines 205. The transparent conductive film is removed partly so that any short-circuit which occurs between the support and the transparent conductive film when the photovoltaic device 201 is cut along its periphery does not adversely affect the effective light-receiving region of the photovoltaic device 201.
A collector electrode 202 for collecting generated electric power in good efficiency is formed on the surface of the photovoltaic device 201. In the case of the amorphous type photovoltaic device, the collector electrode 202 commonly makes use of a conductive ink comprised of a polymeric material formable at a relatively low temperature 201. In the present embodiment, to form the collector electrode 202, a conductive adhesive is provided around a wire formed of copper.
The photovoltaic device 201 thus produced can not be used as such for the generation of electricity. That is, it is necessary to form a terminal through which the generated electric power is led to a means for consuming or storing it. Alternatively, since a single power-generating cell usually has too low a generated voltage, it is necessary to form terminals for making voltage higher by connecting cells in series. Accordingly, an insulating member 204 is provided to ensure insulation between the substrate having a possibility of being laid bare to the outer edges of the photovoltaic device 201 and the electrode layer in the region lying outside the etching line 205 and whose performance is not secured. Then, an about 100 .mu.m thick foil-like terminal member 203 made of a metal is connected to the collector electrode 202 using a conductive adhesive so that it can be used as a power-withdrawing terminal or a terminal for connecting in series another adjoining photovoltaic device constituted similarly.
How to connect the above photovoltaic device will be described below specifically. The above photovoltaic device can achieve materialize, e.g., an optimum operating voltage of 1.5 V and an optimum operating current of 1 A, i.e., an optimum output of 1.5 W under sunlight of AM-1.5.
When ten photovoltaic devices having such output power are used to constitute a module of 15 W, in an extreme case the following output characteristics are obtained. One is a series connection system, where an output with a high voltage and a low electric current can be obtained. In the case of a 15 W module, it is 15 V and 1 A. The other is a parallel connection system, where an output with a low voltage and a high electric current can be obtained, which is 1.5 V and 10 A. Of course, the series connection system and the parallel connection system may be combined appropriately so that intermediate output characteristics can be obtained.
FIGS. 2A and 2B are views showing devices connected in series. FIG. 2A is a diagrammatic plan view, and FIG. 2B a diagrammatic cross-sectional view. In FIGS. 2A and 2B, reference numeral 203 denotes a terminal member, which is a metallic foil member with a thickness of about 100 .mu.m. After an insulating member 204 is provided to ensure insulation between the substrate having a possibility of being laid bare to the outer edges of the photovoltaic device 201 and the electrode layer in the region lying outside the etching line 205 and whose performance is not secured, the terminal member 203 is connected to a collector electrode 202 and is led outside the light-receiving region of the photovoltaic device 201. Thereafter, one end of the terminal member 203 is connected to the backside of an adjoining photovoltaic device 201 by using a solder 307. Thus the series connection is completed.
A crystal type photovoltaic device will be described below.
FIG. 3 is a diagrammatic plan view showing an example of how a terminal is led out of a single-crystal or polycrystalline, crystal type photovoltaic device. In FIG. 3, reference numeral 401 denotes a crystal silicon photovoltaic device, which is a semiconductor layer doped with boron ions on its bottom side and phosphorus ions on the topside. On the lower part of the semiconductor layer, an aluminum paste is coated as a back reflection layer and, on the further lower part of the aluminum paste, a silver paste is coated as a back electrode. On the still further lower part of the silver paste, a solder layer is superposed.
On the top of the semiconductor layer, a transparent electrode layer is formed for the purposes of preventing reflection and collecting electricity and, on the further upper part thereof, a sintered silver paste is formed. On the top thereof, a solder layer is further formed. In FIG. 3, the silver paste and the solder layer are depicted generically as a collector electrode 402. In the present embodiment, the collector electrode has such a form that, as shown in FIG. 3, it has a relatively wide linear land 402a at the middle of comb teeth extending to both sides. Also, on the land 402a, a member made of a metal and having substantially the same width as the land 402a is joined by soldering to form a terminal member 403.
FIGS. 4A and 4B are views showing devices comprising the above crystal type photovoltaic device connected in series. The terminal member 403 is connected with the collector electrode 402 on the land 402a and is outside the light-receiving region of the photovoltaic device 401. Thereafter, one end of the terminal member 403 is put around to the backside of an adjoining photovoltaic device 401 and connected thereto by soldering. Thus the series connection is completed.
However, the photovoltaic devices connected electrically in the above conventional manner require great care in handling.
More specifically, when a group of devices are moved to the next process line after series connection has been completed or when a group of devices are turned over to lead out a final terminal from the back, almost all stress may necessarily be applied to the terminal member 203 or 403 for handling. In such instances, the terminal member 203 or 403 is folded mostly at its edges 305 or 504 to have folds in some cases. As a result, the terminal member 203 or 403 having the folds thus formed comes to have so extremely low a strength that the stress may localize at the folded portions when repeated flexural stress is applied, thereby causing a break.
The above problem can not occur if the terminal member 203 or 403 is a member tough enough to withstand the stress. However, in such an instance, the terminal member 203 or 403 necessarily has such a thick shape that, when the solar cell is sealed later with a filler in order to improve weatherability, air bubbles occur at step portions.
In recent years, as a form of actual use of photovoltaic devices, the development of photovoltaic device modules suitable for installation on roofs of houses is practical and is considered very important.
Photovoltaic device modules installed outdoors are required to have environmental durability. In particular, in the case of amorphous photovoltaic device modules having a flexibility, a repeated flexural load may be applied to the modules when exposed to wind and rain.
The magnitude of such a repeated flexural load differs depending on the size of modules and the manner of installation. Usually, the stress may mostly localize at the connecting portions to cause cracks especially at the points coming into contact with the edge portions of photovoltaic devices, resulting in a break in some cases.